1. Field of the Invention
The present invention relates to an apparatus for controlling an air fuel ratio in an internal combustion engine.
2. Description of the Related Art
In a known device for controlling an air-fuel ratio in an internal combustion engine, an air-fuel ratio is controlled to a value in a lean zone when the engine is operating in a steady state condition, and to a theoretical air-fuel ratio or a value in a rich zone when the engine is under an accelerating condition. This type of air-fuel ratio control system is used to obtain both an increased fuel consumption efficiency and an increased engine output power, which are contradictory requirements.
In such a system, a change from a steady state condition to an accelerating condition is detected, and a setting of a value of an air-fuel ratio is switched from a lean side value to a rich side value. To detect a timing of the switching of the setting from an air-fuel ratio on the lean side to an air-fuel ratio on the rich side, engine load parameters such as an engine intake pressure and/or degree of opening of the throttle valve are detected, and rates of changes in the values of these engine load parameters are calculated. When an acceleration is determined from a large rate of change in the engine load, the air fuel ratio is switched from a lean setting to a rich setting, wherein the air-fuel ratio is equal to a theoretical air-fuel ratio or an air-fuel ratio on the richer side. To detect an acceleration condition, usually the rate of change of the degree of opening of the throttle valve is calculated, and then it is determined whether the rate of change of the degree of opening of the throttle valve is larger than a predetermined threshold value, and thus it is determined if a switch from the steady state condition to the acceleration condition, and vice versa, has occurred. To obtain a value of the air-fuel ratio which matches an acceleration requirement of the driver, not only a rapid acceleration but also a very slight acceleration must be precisely detected, and thus the air-fuel ratio is controlled in accordance with the detected degree of acceleration. Nevertheless, a precise and rapid detection of a mild acceleration is very difficult, for the following reason. To realize a rapid detection of the acceleration, it is necessary to shorten the timing at which the value of the degree of opening of the throttle valve is read as a parameter of the degree of load on the engine. Furthermore, to detect a very mild acceleration condition, it is necessary to lower the value of the threshold of the rate of change of the degree of the opening of the throttle valve, to determine the acceleration. Nevertheless, as well known, a steady state condition in an internal combustion engine does not always require a constant degree of opening of the throttle valve. Namely, small changes in the value of the degree of opening of the throttle valve frequently occur even when the engine is running under a steady state condition. This means that a small threshold value intended for a detection of the slightest acceleration makes it difficult to differentiate that acceleration from a change in the degree of opening of the throttle valve generated during the steady state condition, and thus the calculated speed of the change of the degree of opening of the throttle valve will exceed the threshold valve even if the engine is not in an acceleration condition but a steady state. This causes the setting of the air-fuel ratio to be changed to that for a high engine output, regardless of the driver's intention to maintain a steady state running, and such a rapid increase in the engine output power causes the driver to feel that the engine is in a state in which it should not be. The setting of a sufficiently large value of the threshold of the speed of change of the degree of opening of the throttle valve, to prevent this from happening, makes it impossible to detect a mild acceleration as desired, and thus the air-fuel ratio is not properly controlled.
Japanese Unexamined Patent Publication No. 63-129140 discloses a counter which is incremented when a value of a rate of change of the degree of opening of the throttle valve exceeds a predetermined value, and which is cleared to zero when the value of the rate of the change of the degree of opening of the throttle valve falls below the predetermined value. The acceleration operation is differentiated from the steady state condition by determining whether or not the value of the counter exceeds a predetermined value.
Nevertheless, this prior art of Japanese Unexamined Patent Publication No. 63-129140 suffers from a drawback in that a quick detection of a mild acceleration is difficult, for the following reason. When the engine is under a mild acceleration condition, the condition at which the rate of change of the degree of the opening of the throttle valve is larger than a predetermined value is not always maintained, and therefore, a situation may frequently occur wherein a rate of change of the degree of opening of the throttle valve falls below said predetermined value, due, for example, to the occurrence of noise by which the counter is cleared. As a result, it is difficult for the counter to obtain the threshold value and determine an acceleration, causing a delay in the detection of the acceleration. This means that there will be a delay in a change in the setting of the air-fuel ratio with respect to the mild acceleration requirement by the driver, causing the engine output torque to be "unnaturally" changed and worsening the driveability.